In addition to providing insights into the mechanism of viral entry, the identification of chemokine receptors as coreceptors of HIV may explain certain questions related to HIV transmission and AIDS pathogenesis. The importance of the CCR5 coreceptor in viral transmission is underscored by the observation that individuals who lack a functional CCR5 protein are largely protected from HIV-1 infection. Furthermore, regardless of the route of infection, R5 viruses are preferentially isolated from HIV-1 infected individuals during primary and acute infection. Despite the dominant role of R5 viruses in HIV-1 transmission and early infection, in about 50 percent of patients, CXCR4 using (X4) viruses emerge during late infection. The outgrowth of X4 viruses coincides with a loss in circulating CD4+ T cells and precedes the onset of AIDS. Indeed, studies in several in vitro and ex vivo tissue culture model systems, in addition to SCID-hu mouse models, revealed that X4 viruses are more cytopathic. Based on these findings, it has been widely presumed that R5 viral variants are selectively transmitted by sexual contact and that the phenotypic switch during HIV infection is the casual factor in acquired immunodeficiency. However, recent studies in the macaque model call into question this central belief. Using infection of rhesus macaques with SHIVs expressing R5 or X4 specific HIV-1 envelope as a model system, we find that both R5 (SHIVSF162P) and X4 (SHIVSF33A) SHIVs can establish systemic infection by the intravaginal (IVAG) route. Furthermore, both viruses are cytopathic, but they cause CD4+ T cell depletion in distinct anatomical compartment. Since these viruses appear to replicate with comparable kinetics and to similar titers in intravenous (IV) and IVAG infected animals, they provide the ideal setting for addressing the role of coreceptors in HIV2 transmission and pathogenesis. We propose that differential viral dissemination, rather that the ability of the virus to cross the mucosal barrier, is the major determinant for the apparent preferential transmission and early infection of R5 viruses. Furthermore, we hypothesize that AIDS pathogenesis will e influenced by coreceptor usage. Four specific aims are proposed. (1) Compare the properties of the X4-specific SHIVSF33A and R5-specific SHIVSF162P in vitro. The replication kinetics, tropism and cytopathicity of the viruses will be determined. (2) Assess the role of coreceptor usage in defining the sites of virus replication and disease course in IV and IVAG inoculated animals. Viral loads, viral set points, compartments of virus replication and anti-viral immune responses in the infected animals will be assessed. T and B cell subset distribution in the bone marrow, gut, thymus, peripheral blood and lymph nodes will also be determined. (3) Perform IV and IVAG infection with mix inocula to determine whether there is preferential transmission of R5-utilizing virus and, if so, whether this preferential transmission is dependent on the route of infection. (4) Identify the first site and target cell of virus replication, and characterize the route of viral dissemination in R5 and X4 infected animals.